1. Field of the Invention
The invention is a process for preparing chemically active solid oxide particles useful for polishing, composed primarily of CeO.sub.2, or CeO.sub.2 together with other oxides, which consists of mixing an aqueous solution of a water soluble trivalent cerium salt together with an oxidizing agent and aging said mixture in the liquid state for a time not less than 4 hours.
2. Description of the Prior Art
The invention is useful for the polishing of glass and other silica or silicon containing articles. The polishing process consists of the controlled abrasion of a solid surface by a suspension of small solid abrasive particles which is pressed against the substrate surface by a rotating viscoelastic pad. While, in principle, slurries of any particles may be used to polish a surface, in practice, only specific classes of particles have been shown to exhibit polishing rates which are high enough to be economically useful. Chemically active polishing compounds are restricted to a small class of oxides, particularly, CeO.sub.2, ZrO.sub.2, Fe.sub.2 O.sub.3 and SnO.sub.2. Of these oxides, CeO.sub.2 has by far the highest polishing activity, and is the preferred basis of the vast majority of commercially available polishing slurries.
The above oxides have been shown to require specific activation sequences before high rates of polishing are observed. Specifically, all known prior art polishing compounds require calcination, i.e. a roasting of previously prepared solid particles at some temperature below the sintering point of the oxide, to produce the desired activity. While it is recognized that a partial reason for the calcination is the conversion of solid starting compounds (e.g. carbonates, oxalates, nitrates or sulphates) to the corresponding oxide, specific sintering temperatures not directly related to the conversion have been demonstrated to have a pronounced effect on rate.
While calcination is universally practiced in the manufacture of polishing compounds, it is disadvantageous for several reasons. First, gaseous byproducts are produced which in some cases can be toxic or corrosive (CO, NO.sub.x, HF, SO.sub.3). Second, calcination is energy intensive and relatively expensive. Third, exposure to furnace environments leads to the contamination of the polishing compound by furnace refractories or environmental debris which may degrade polishing performance due to scratch formation by the foreign material.
The most significant difficulty caused by the calcination process is that products must be milled after calcination to yield particle size distributions in the final product which are sufficiently small to prevent scratching. Particles above a critical diameter produce a scratched rather than a polished surface. While this is less critical for coarse polishing processes, production of critical optical surfaces, semiconductor wafers and integrated circuits requires a complete freedom from surface defects, achievable only when particle size distributions fall below 1 micron. Production of such distributions by conventional milling techniques is extremely difficult and often not economically feasible.
To date, the only technique which has been shown to produce solid oxides directly from solution is hydrothermal synthesis. In this process, amorphous metal hydrous oxides are held at elevated temperature and pressure for varying times to produce small particles of the solid oxide in the final solution. While the process has been extensively investigated for use in structural ceramics as in U.S. Pat. No. 4,619,817, there has been no indication of its utility in preparing polishing compounds. To date, only two publications have disclosed methods for preparing CeO.sub.2 particles via hydrothermal synthesis. They are:
Hsu et al, (Langmuir vol. 4, pp.31-7, 1988), aged extremely dilute (10.sup.-4 molar) solutions of ceric sulphate at low pH (0.5-2.0) at 90.degree. C. to yield submicron particles of cubic CeO.sub.2 which were contaminated with varying amounts of sulphate. No effort was made to evaluate polishing activity. The extremely low starting concentrations make such a synthesis economically infeasible.
Grahl-Madsen and Riman (Proc. 3rd Int. Conf. on Ceramic Powder Science, pp. 33-40, 1990) described the incorporation of cerium into zirconium oxide powders prepared via hydrothermal synthesis. Trivalent Cerium salts, Ce(NO.sub.3).sub.3, were added to a basic (pH 9.5) solution of 1 molar Zr(NO.sub.3).sub.2 *XH.sub.2 O to form a gel which was then exposed to hydrothermal conditions (closed cell at 250.degree. C.) for varying times so as to transform the gel into a crystalline oxide product. Pure cerium oxides were prepared using extremely dilute (0.016M) starting solutions of Ce(NO.sub.3).sub.3. Cubic non-stoichiometric CeO.sub.2 products were produced which had a primary crystallite size of 40 nm. The reaction mechanism proposed was the hydrothermal transformation of an amorphous hydrous oxide gel precursor into a solid crystalline oxide product. No polishing data were disclosed.
From the above, it is clear that a technique which produces solid oxide polishing compounds of appropriate size and polishing activity directly from solution at high solution concentration would be highly desirable, as almost all of the manufacturing steps currently required for production could be eliminated.